Usually, an ophthalmic lens comprises a visual correction that is determined by a prescription established for the wearer of the lens. Such a prescription indicates in particular a value for the optical power and a value for the astigmatism adapted to correct the distance vision of a wearer. These values are usually obtained by combining the anterior face of the lens with a generally spherical or spherotoric posterior face. For a progressive lens, at least one of the two faces of the lens has spherical and cylindrical variations, resulting from variations in optical power and astigmatism, between different observation directions through the lens. The type of variations of optical power and astigmatism of the lens are called design. In particular, the difference in optical power between two points dedicated to distance vision and to near vision is called addition, and its value should also correspond to the value prescribed for a far-sighted wearer.
Currently, a progressive lens is produced in two successive steps. The first step consists of producing a semifinished lens, of which the anterior face has spherical and cylindrical variations initially defined to correspond to the desired design. This is performed in a factory, for example by molding or injection. Semifinished lenses are divided into several models which can differ, in particular, by the base, by the distribution of sphere and cylinder of the anterior face, or by addition. The base is the sphere at the point on the lens corresponding to distance vision. The distance between the near vision and distance vision points, the respective widths of the zones of the lens corresponding to near vision and distance vision, the refractive index of the transparent material that constitutes the semifinished lens etc, can also differ from one model to another. Each combination of these characteristics corresponds to a different model of semifinished lens.
The second step is performed in laboratories located between factory and retail sales center within the distribution chain for ophthalmic lenses. It consists of machining in a separate step a sphere or a spherotoric surface on the posterior face of the lenses, so that each lens corresponds to the prescription of the wearer.
At the present time, a trend has appeared whereby the design of progressive lenses is customized according to supplementary characteristics of the wearer, other than the usual prescription characteristics. Such supplementary characteristics may concern in particular the position of the head of the wearer and that of his eyes for a distance vision situation and a near vision situation. The progressive lens may then, for example, be selected so that the distance vision and near vision points are situated at locations in the lens that are adapted in relation to the positions of the head and eyes of the wearer.
In the organization for producing progressive lens described above, taking into account individual wearers' characteristics for the design of lenses, requires a multiplication of models of semifinished lenses. The series of semifinished lenses for each model that are produced in a factory are then shorter. Their cost price is consequently higher. Moreover, this results in complex stock management in the laboratories, since these must have reserves available for a large number of models of semifinished lenses.
In order to avoid such multiplication of models of semifinished lenses, a novel organization of the production line for progressive ophthalmic lenses has been proposed. According to this novel organization, the design of the progressive lens is provided by the posterior face of the lens. Semifinished lenses then possess an anterior spherical face and the posterior face is machined subsequently according to the prescription and to the design that is adapted to the individual characteristics determined for each wearer. Such an organization is particularly flexible, given that any individual characteristic of the wearer is no longer involved in the selection of the semifinished lens model. In particular, a smaller number of semifinished lens models is sufficient for obtaining all the configurations of finished lenses.
However, in this case, the posterior face of the lens possesses a complex form. Indeed, the design and correction result together from this form. Subsequent machining of the posterior face of the semifinished lens then requires that laboratories be equipped with machines capable of producing such shapes. Such machines, which correspond to what is called a “free-form” method, are themselves complex and therefore costly. For these reasons, subsequent machining of the posterior faces of lenses should be grouped together in a restricted number of specialized laboratories, which goes contrary to customization of lenses moved downstream in the production and distribution chain.
Document U.S. Pat. No. 5,444,503 describes a progressive ophthalmic lens with an anterior face which can be a progressive surface and a posterior face which is formed not only in order to obtain the prescribed correction for the wearer, but also in order to take account of individual conditions of use of the lens. These individual conditions of use include the depth of the eyes, the distance of vision, the inclination of the lens in front of the eye according to the frame into which the lens is fitted, and the shape of the frame. The posterior face of the lens may then also be aspheric or atoric. Customization of the lens is therefore achieved, in addition to the correction of ametropia, without increasing the number of semifinished lens models. However, such customization only takes account of the physical characteristics of the eye and/or of the frame. Now, such characteristics are insufficient for procuring an improvement in the comfort of the wearer under many conditions in which the lens is used.
An object of the present invention is therefore to combine economical production of progressive lenses with customization of the design of each lens according to at least one individual characteristic of the wearer other than his prescription, while procuring an improvement in the comfort of the wearer under a large number of conditions of use.